1. Field of the Invention
The invention relates to an electrochemical storage cell based on sodium and sulfur with a anode space and a cathode space, which are separated from one another by a solid electrolyte and are defined at least in some areas by a metal housing, wherein the cathode space, in order to form an electrode, is filled with filamentary material of graphite or carbon that is saturated with sulfur.
2. Description of the Prior Art
Such electrochemical storage cells are suitable as energy sources. They are increasingly used in designing storage batteries that are intended for supplying current to electric vehicles.
One special example of these storage cells is those cells based on sodium and sulfur, which are rechargeable and have a solid electrolyte of beta-aluminum oxide, which separates the anode space from the cathode space. One pronounced advantage of these storage cells is that when they are charged, secondary electrochemical reactions do not take place, and the current yield is approximately 100%. In such storage cells, the anode space is filled with sodium and disposed inside the cup-shaped solid electrolyte. The cathode space is located between the solid electrolyte and the metal housing, which defines the storage cell with respect to the outside. Inside the cathode space, in storage cells known until now, a long-filament material of graphite or carbon is provided, which to form the electrode is saturated with sulfur. When the storage cells are produced, half-shell-like elements are shaped from the the filamentary material, saturated with sulfur and then inserted into the cathode space. The storage cells are manufactured at room temperature. For operation, the storage cells are heated to a temperature of 350.degree. C. If a storage cell is exposed to this kind of temperature influence, the result is an expansion of the filamentary material, and in particular of the two half-shells that are disposed in the cathode space. They expand to such an extent that their face ends are flush with one another, and the fibers of one half extend into the fibers of the other half such that no space remains in the boundary area of the half shells. When the storage cells are discharged, the sodium ions contained in the anode space pass through the solid electrolyte into the cathode space, where they form sodium polysulfide with the sulfur provided there. Because the half-shells formed from the long-filament material now touch one another closely, the sodium polysulfide can be uniformly distributed, in particular via the boundary spaces of the two half-shells, in the cathode space. If such a storage cell containing relatively large quantities of sodium polysulfide in the cathode space is cooled to a temperature below 280.degree. C., then it solidifies into a closed ring that firmly surrounds the solid electrolyte. The sodium polysulfide has a higher thermal coefficient of expansion than the beta-aluminum oxide from which the solid electrolyte is made. This means that the ring formed from the sodium polysulfide shrinks onto the solid electrolyte when it cools. As a result, it adheres very firmly to the outer surfaces of the solid electrolyte, and with decreasing temperature exerts such force upon the electrolyte that breakage of the solid electrolyte can result.